[tz] switches at 1883-11-18 at the same instant

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Tue Mar 20 04:56:47 UTC 2018

On 2018-03-19 16:51, Paul Eggert wrote:
> On 03/19/2018 03:09 PM, Michael H Deckers via tz wrote:
>> the switch was done at a single instant, when UT was 1883-11-18 + 18 h. 
>> This differs from the current description in tzdb which uses different
>> instants for different target time zones -- not a likely choice for
>> simplifying a nationwide railway schedule.

> But we have a reliable eyewitness account that New York had two noons that day.
> See the quotation from William F. Allen in the "northamerica" file, taken from
> Bartky's 1989 paper <http://dx.doi.org/10.2307/3105430>. So the New York switch
> cannot have been at 18:00 GMT that day, as that would have meant two instances
> of 13:00, not two instances of 12:00. Bartky writes that it was called the "day
> of two noons" because eastern parts of the new zones observed noon twice, which
> wouldn't have happened if everyone switched at 18:00 GMT.
> Allen also wrote that New York got its time signal from the Naval Observatory,
> not from the Allegheny Observatory. Back then, different observatories were
> competing for the time-setting business. Perhaps the SpaceWatchtower sources
> included Allegheny partisans? (It's hard to tell from its source list.)
> It's possible that parts of the US switched at around 12:00 local time while
> other parts switched at 18:00 GMT; but if that's the case, I'd like sources for
> which parts switched which way.

Civil time zone acceptance and standardization often lagged railway time zone
standardization by years. A recent post documented Paris railway station clocks
showing railway time inside and civil time outside 5 minutes apart IIRC. Clock
towers in a number of locations are documented having two minute hands to show
both local and standard times, until using standard time became a commonly
accepted practice in that locale.

The time zone db documents common civil practice not industry practices; for
example, North American natural gas flows according to industry practice in a
year starting Nov 1 and day starting 09.00 CT: time zones in North America would
be greatly simplified if every industry followed this model and used CT. ;^>

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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